Liquid- or gas-sensors are used for measuring pH-value or redox potential, temperature, conductivity, or turbidity in liquids.
In the following, the discussion will essentially concern potentiometric sensors, as an example for a liquid- or gas-sensor.
Potentiometric sensors are mostly used for determining potentials in the case of high resistances, such as is the case with pH-measurements and redox measurements. With the help of pH-electrodes, or redox electrodes, the potentials of the solutions are registered.
In many applications, these electrodes are exposed to heavy wear, such that they frequently must be replaced after a short operating time.
There are very simply constructed pH-sensors, which are made up of only one pH-electrode without any other electronic components. These pH-electrodes deliver a pH-dependent potential, which can be tapped at suitable electrical connections. Optionally, for temperature compensation, these pH-electrodes have an integrated temperature sensor, e.g. PT100, the potential of which can be tapped at suitable temperature outputs. For measuring, these pH-sensors are normally connected via a cable to a transmitter, which generates a measurement signal from the pH-dependent potential and, if necessary, from the temperature signal of the temperature sensor.
Besides the simple pH-electrodes, or sensors, described, there are also such with integrated preamplifiers for impedance conversion. The output signal of the preamplifier is the potential of the pH-sensor, with, however, instead of the internal resistance of the pH-sensor, which lies in the order of magnitude of 100 MΩ, the internal resistance of the preamplifier of a few Ω now being determinative. Therefore, the further transfer and processing of the output potential is greatly simplified for a transmitter. The preamplifier is supplied with voltage either via a battery or via a cable.
Furthermore, simple transmitters, which are mounted directly on the pH-sensors, are available under the name “Direct Line” of the firm, Honeywell. These make it possible to generate, right at the sensor, e.g. a 4-20 mA measurement signal, which can then be transferred directly to a control station.
In the case of all known pH-electrodes, or pH-sensors, it is necessary to calibrate the electrodes after connection to the transmitter, in order to be able to store the ascertained calibration parameters in the transmitter. Sensor-specific information, such as the designation of the measuring point, etc., is normally not available on-site, that is, in the direct the vicinity of the sensor.
The sensor system offered by the firm, Endress+Hauser, under the mark, MEMOSENS, includes a sensor module and a sensor module head, which can be plugged together. Data exchange between sensor module and sensor module head, and energy supply to the sensor module, are accomplished inductively via a connecting section, which serves for galvanic decoupling. Furthermore, a sensor module is provided in the digital memory, in which, among other things, calibration parameters are stored. This technology is disclosed, for example, in the German (published applications) DE 100 55 090 and DE 102 18 606.
As an accessory to a sensor as disclosed therein, German Offenlegungsschrift DE 103 44 262 discloses a plug-in module, which especially enables sensor-specific information to be represented on-site, directly at the sensor, and furthermore enables a checking of the measurement values sent to the control station. In a special embodiment, this module is not only suited to display data from the sensor, but also to transfer such to a superordinated unit, per cable or per radio.
In certain applications, data transfer per radio requires an energy expenditure too large to allow sustained operation of the sensor by means of a plug-in module. On the other hand, operation of the sensor by means of a cable, which runs from a measuring transmitter to the sensor, is not always practical, especially in the case of remotely-positioned sensors.